This invention is drawn to the field of sub-micron scale thin film micro-fabrication techniques, and more particularly, to a novel method for fabricating Josephson weak-links employing electron beam lithography.
Non-hysteretic weak-links such as the variable thickness microbridge (VTB) and the SNS junction utilize the AC Josephson effect to produce tunable radiation in the microwave and millimeter wave regions of the electomagnetic spectrum. Reference in this connection may be had to co-pending applications Ser. No. 258,705, filed Apr. 29, 1981, U.S. Pat. No. 4,470,023 and to Ser. No. 258,704, filed Apr. 29, 1981, U.S. Pat. No. 4,468,635, both of which are incorporated herein by reference, which show and describe means and methods for combining superconducting weak-links coherently to produce radiation the linewidth of which scales as 1/N and the power of which scales as N.sup.2, where N is the number of coherently arrayed junctions.
A photolithographic method, described by Dolan in an article entitled "Offset masks for lift-off photoprocessing", appearing in Volume 31, No. 5, Appl. Phys. Lett., pp. 337-339, (1977), incorporated herein by reference, uses masks offset from the substrate and oblique angle thin film deposition to fabricate a VTB and other superconducting devices. A bi-level mask consisting of a support layer of resist having a thickness .delta..sub.1 is overlayed by a second layer of resist having a comparable thickness .delta..sub.2. The first layer of the resist is undercut and the overlying second layer of resist contains the pattern information. A second layer of resist in the form of a suspended bar as shown in FIG. 3(a) can be used with two depositions from opposing oblique angles to form a wide varying thickness superconducting microbridge. FIG. 4(a) shows that a second layer of resist in the form of a suspended bar having a break can be used with oblique angle depositon to form a narrow varying thickness microbridge. Reference may be had to an article entitled "Self-aligned thin film structures with 1000 .ANG. resolution", by Howard et al, appearing in Vol. 36, No. 2, Appl. Phys. Lett, pp. 141-143, (1980), incorporated herein by reference, which shows that e-beam lithography can be utilized with such bi-level shadow masks to produce devices having line resolution an order of magnitude better than that obtained with the optical methods.